Various types of arc discharge lamps have been proposed, in which a single arc vessel or discharge vessel retains electrodes within which an arc can be struck, the vessel also retaining a fill which includes mercury, a noble gas, and rare-earth halides. Typically, the rare-earth halides include holmium and/or dysprosium. Such lamps have a specific wall loading of between 50 to 120 W/cm.sup.2, and a specific arc power of between 1 kW and 5 kW/cm.
U.S. Pat. No. 3,654,506 and British Pat. No. 1 376 509 describe high-pressure discharge lamps retaining mercury vapor within the discharge vessel--when in operation--and additionally rare-earth halides, particularly halides of dysprosium and/or holmium and/or thulium. These lamps have a light output of over 85 lm/W. The light output, with respect to spectral distribution, of such lamps should approach daylight as closely as possible. Consequently, the color should be about 6000.degree. K., with a color rendering index Ra of 90 or better. Lamps of this type are used particularly for illumination of stages for theatrical and similar performances, for film studios, television studios and the like, or in any other installation where the color temperature should approach that of daylight as closely as possible, and where excellent color rendition is important.
Lamps of this type which have a rated power of below about 4 kW can be made, and which also have the desired color spectral distribution of the light output. It has been found, however, that as the power of such lamps is increased, with the same composition of the halides, the color temperature of the light decreases markedly, so that the spectral composition of the light output will no longer satisfy the requirements placed on such lamps.
A color temperature of about 6000.degree. K. is necessary since the actual output of the light changes somewhat when the lamps are installed in suitable lamp fixtures or illumination systems. Installing such lamps in illumination systems or fixtures causes the color temperature of the emitted light to drop by several 100.degree. K. In order to obtain good color balance and good color rendition of scenes illuminated by the lamps used for film recording, for television use or the like, it is necessary that the illuminating light have a color temperature of about 5600.degree. K. Consequently, the lamps by themselves, external of an entire illumination system and/or illuminating light fixtures and the like, should have a color temperature which is approximately in the order of 6000.degree. K.